Heat engine

In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred to the sink from the source, and in this process some of the heat is converted into work.

Efficiency

The efficiency of a heat engine relates how much useful power is output for a given amount of heat energy input.

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From the laws of thermodynamics:

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:: E_W = E_H - E_C

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:where

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:: E_W is the useful energy from the engine.

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:: E_H is the heat energy taken from the high temperature system

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:: E_C is the heat energy delivered to the cold temperature system

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In other words a heat engine absorbs heat energy from the high temperature heat source, converting part of it to useful work and delivering the rest to the cold temperature heat sink.

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The efficiency of a given heat engine is defined by:

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::e = rac{E_W}{E_H} = 1 - rac{E_C}{E_H}

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The theoretical maximum efficiency of any heat engine depends only on the temperatures it operates between. This efficiency is usually derived using an ideal imaginary heat engine such as the carnot heat engine, although other engines using different cycles can also attain maximum efficiency. This efficiency is:

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::e = rac{T_h - T_c}{T_h} equiv 1 - rac{T_c}{T_h}

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where T_h is the absolute temperature of the hot source and T_c that of the cold sink, usually measured in kelvins.

Related Topics:
Absolute temperature - Kelvin

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The reasoning behind the proof of this theorem relates to the laws of thermodynamics. It is first assumed that if a more efficient heat engine than a Carnot engine is possible, then it could be driven in reverse as a heat pump. Mathematical analysis can be used to show that this assumed combination would result in a net decrease in entropy. Since no exceptions have ever been found to the Laws of Thermodynamics (which require that entropy for a closed system never decreases) it is concluded that it is not possible to build a heat engine more efficient than a Carnot Cycle engine.

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Empirically, no engine has ever been shown to run at a greater efficiency than a Carnot Cycle heat engine.

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